For a vehicle having an automatic transmission apparatus, there has been known a system of a type in which a target speed or acceleration of the vehicle is set according to the position of an acceleration pedal, and a throttle valve disposed in an air intake passage of an engine (or a fuel injection control governer for a diesel engine) is electronically controlled so that the target speed is reached. There has also been known an automatic speed control system that can set a target speed of the vehicle manually, in place of referring to the position of the acceleration pedal. In these prior art systems, the target speed and a measured speed of the vehicle have been compared and the vehicle speed has been feedback-controlled, but the feedback gain factor has not been changed according to the running condition of the vehicle.
With the above prior art systems, when a vehicle running at a constant speed enters an ascent in the road, the vehicle speed decreases and then, by the function of the feedback control, restores the original speed. However, the time for the restoration of the original speed depends on the gradient of the ascent and the number of passengers carried. When the feedback gain factor of the control system is increased, the response can be improved; however, this results in a hunting or an overshooting because the vehicle is an object of control with a substantial delay time. When the feedback gain factor of the control system is decreased, such a hunting or an overshooting can be prevented; however, this results in a defective response.
With a view to obviate all of the prior art defects of running control systems for a vehicle, it is a primary object of the present invention to provide a running control method and a system therefor, in which the feedback gain is controlled according to effects of slope, wind, and weight on the vehicle.